0.9.9-delta

Fix Issue #72

• A possible reason for this issue is due to multi-threading problem.
• When flushing db files, a db session is closed at first; however, still there are some threads that want to write data into a the db.
• To prevent such a problem, the previous implementation locked each write-operations with mutex.
• However, when writing multiple entities, I found that ObjectBox actually put entities one by one, not at once.
• Therefore, each write-operation is not thread-safe when writing multiple entities.
• This version allow to write only one data with mutex.